Measurement op mechanical resonant frequency of steel bodies - using electromagnet coil to detect magnetic field changes when body vibrates within it

摘要

Measuring accurately and rapidly the mechanical resonant frequency of a small steel body, partic. of cylindrical form, is carried out by an electromagnetic coil mounted round the body to detect the sinusoidal changes in a permanent magnetic field through the coil when the body is pulsed into vibration. Basically, a permanent magnet (2) generates a field which passes through a pickup coil (3) on the axis of which the test body (1) is mounted. A hammer (5) operated by an electromagnet (6) pulses the body (1) and produces a damped vibration, which in turn produces a signal from the pick-up coil (3). This is amplified, filtered, re-amplified, squared and applied to a timing and display network based on a quartz crystal oscillator. A result accurate to one part in a million can by attained.